A magic system with clearly defined rules, costs, and limitations that the reader understands well enough to predict outcomes.
A hard magic system is one where the rules of magic are explicit and consistent. Readers know what the magic can do, what it costs, and where its boundaries are. This transparency means characters can solve problems with magic without it feeling like cheating, because the audience understands the constraints. The "hardness" isn't about rigidity; it's about how much the reader is let in on the rules.
Hard magic systems let you use magic as a problem-solving tool in your plot without losing tension. When readers understand the rules, a clever magical solution feels earned rather than pulled from thin air. If you want magic to drive your climax, it probably needs to be at least somewhat hard.
Allomancy is the textbook hard magic system. Each metal does one thing, and the reader learns the rules alongside the protagonist.
Alchemy follows the law of equivalent exchange: you can't create something from nothing. This single rule drives the entire story's conflict.
Chromaturgy converts light of specific colors into a physical substance called luxin, with each color having defined properties and costs.
Start with 2-3 core rules and build complexity gradually. If you need a glossary to explain your magic, it's probably too dense for the opening chapters.
Rules should create dilemmas, not just mechanics. The best hard magic forces characters into meaningful choices.
Create a hard magic system with exactly three rules written in plain English. Then write a scene where your protagonist faces a locked door they must get through. They can only use this magic system to do it, and at least one rule must make the task harder. Focus on how the constraints shape the solution.